1. Field of the Invention
The present invention relates to tougheners for epoxy resin adhesives, to epoxy adhesives, as well as to their manufacture and use, and to articles comprising the tougheners and/or adhesives.
2. Discussion of Background Information
Epoxy-based adhesive compositions are reactive adhesive compositions comprising an epoxy resin, a curing agent and, usually, an accelerator. Upon heat-activation, epoxy groups of the epoxy resin react with the curing agent thereby linking the epoxy resin compounds by a polyaddition reaction to obtain a cured product. Such a cured product is known to have good mechanical properties and a chemical resistance superior to the cured product of other reactive adhesives. These characteristics make epoxy adhesive compositions particularly useful for demanding applications where stringent mechanical requirements must be satisfied, for example, in the automotive industry.
The cured product of an epoxy adhesive, other than a structural epoxy adhesive, generally has a relatively high static strength, for example, a high tensile and lap shear strength. Its dynamic strength however, e.g., its impact peel strength, is generally low. Adhesives used for the assembly of parts of a vehicle, such as automobiles, vans, lorries, trucks and trains, are called structural adhesives. The cured product of such a structural adhesive has to bear high static and dynamic loads. For that purpose, additional flexibilizers and/or tougheners are often added to the epoxy adhesive composition.
Structural epoxy adhesives have been described in several patent applications: EP-A-0 197 892 (and U.S. Pat. No. 4,659,779; U.S. Pat. No. 4,713,432: U.S. Pat. No. 4,734,332; U.S. Pat. No. 4,701,378 (all five of which are incorporated herein in their entireties by reference) describe a structural adhesive comprising an epoxy resin, a nitrogen comprising tougher and an accelerator. A problem described by these documents is to provide a structural adhesive having a good storage stability and a high curing rate. EP-A-0 308 664 (and U.S. Pat. No. 5,278,257 (both incorporated herein in their entireties by reference) describe an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyphenol-terminated polyurethane or polyurea. EP-A-0 353 190; (U.S. Pat. No. 5,073,601; U.S. Pat. No. 5,202,390 (all three incorporated herein their entireties by reference) describe an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyether-, polyester, polythioester or polyamide prepolymer terminated with a functionalized carbocyclic aromatic or araliphatic residue. Technology according to EP-A-0 308 664 and EP-A-0 353 190 is called Mulhaupt technology. The problem described by both EP-A-0 308 664 and EP-A-0 353 190 is to improve the impact peel strength of the cured product. At low temperatures, however, the impact peel strength is low. WO 00/20483 and (U.S. Published Patent Application No. 2004/0081013 (both incorporated herein in their entireties by reference) relate to a composition comprising an epoxide-reactive copolymer having a glass transition temperature of −30° C. or less, and a reaction product of a carboxylic acid anhydride with a di- or polyamine and a polyphenol or aminophenol. The cured product of WO 00/20483 has an impact peel strength of less than 20 N/mm at −40° C.
A problem with epoxy resins used as structural adhesive compositions is that the epoxy resins are somewhat brittle and subject to fracturing when impacted. This tendency to fracture can be reduced by the addition of toughening agents (or tougheners). The problem with the use of toughening agents is that such toughening agents tend to increase the viscosity of the composition and the increased viscosity limits the method and speed of application. Mulhaupt, U.S. Pat. No. 5,278,257 (discussed above) discloses an epoxy resin containing (a) copolymer based on at least one 1,3-diene and at least one polar, ethylenically unsaturated comonomer and (b) a phenol-terminated polyurethane, polyurea or polyurea urethane. The adhesives disclosed in Mulhaupt are excellent structural adhesives. These toughening agents are very viscous, and their viscosity tends to increase over time. This can limit their usefulness, e.g., by limiting their shelf-life, or limiting their utility in some high-volume applications which require high application speeds.
Some of the tougheners in the above documents can be generally described by the following idealized structure Formula I:

In Formula I, A is trimethylolpropane (branching unit); B is poly THF (telechelic polymers, hydroxy functional); C is hexamethylenediisocyanate (di-isocyanate); and D is bisphenol A (building block and capping compound).
There is a need for toughening agents that have greater viscosity stability and improved viscosity.